I’m in the process of buying another property and once it closes we’ll have to immediately install new smoke detectors and carbon monoxide detectors which he’ll have to take time from the apartment project to coordinate with the electrician on.

You are not a lie detector and you need to stop trying to figure out who is worthy and who is not.

Radon detector pretty pricey, infa-red camera, are you able to check electrical circuits? 

One good idea is to buy the $10 leak alert detectors that will scream like a fire alarm when they are in contact with water.

In WA It also means carbon monoxide and smoke detectors and water heaters are strapped. 

The first thing I do when I buy a property is update the CO detectors and the smoke detectors.

And of course working smoke alarms (and if gas, carbon monoxide detectors).  

One interesting find during the inspection of the "in law" suite was that no smoke detectors were installed in the furnished basement.That being said, this has me concerned about the potential quality of work that went into the rehab, and whether or not licensed contractors/personnel did the work?

I always conduct a walk-thru of the property every 6-months to check for leaks, fire/ CO detectors, etc.

Other simple things include grounding/reversed polarity of outlets (you can buy an outlet tester at Home Depot for $5), using foil-faced tape at furnace & water heater flue pipe connections, operable smoke/CO detectors, cracked/chipping/peeling paint if your applicant has children under the age of 6 (I think?)